The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Photovoltaic (PV) cells that convert sunlight directly into electricity are becoming increasingly important in the world's renewable energy mix. Currently, around 85% of PV cells have a photoactive element based on crystalline Si, with the rest being polycrystalline thin film PV cells, mostly cadmium telluride/cadmium sulfide ones. Thin-film cells tend to be cheaper to make with a shorter energy payback time. A rapidly developing newcomer to the thin film PV field is based on organic-inorganic perovskite-structured semiconductors, the most common of which is the triiodide (CH3NH3PbI3). Such Perovskites tend to have high charge-carrier mobilities and therefore make ideal photoactive components.
Large scale production of these types of PV cells is difficult because the process of applying the photoactive layer has been found to be difficult to scale.
Vacuum evaporation has been proposed for constructing perovskite thin films utilising co-evaporation of two precursors (PbCl2 and CH3NH3I). Whilst the resulting films exhibit satisfactory perovskite film coverage and uniformity, this technique demands high vacuum, and therefore is very energy intensive in a scaled up process.
Alternatively, solution-based techniques can be used to fabricate thin films, in which a mixture of two precursors is used to form the completed absorber. Spin coating can be used at the lab scale to coat a perovskite precursor solution onto a substrate. Spin coating allows for the formation of large size crystals to be minimized or controlled formation thereby forming very homogenous films over an area up to 300 mm in diameter. However, it is not possible to use spin coating on a larger scale. In a scalable coating/printing process, for example dipping or wet coating, a wet film of Perovskite coating material is formed first and then dried. This has a significantly different solution dynamic and drying time compared to spin coating. In wet coating, the wet film is susceptible to dewetting, non-homogenous crystal formation and/or the formation of pinholes, all of which have undesirable effects on the function of the photoactive layer. These problems become increasingly more serious the greater the amount and area to which the crystalline material is applied.
In yet another method, a metal compound layer is formed on a substrate, for example by electrodeposition (for example a PbO layer in, Cui et al, Electrodeposition of PbO and In Situ Conversion to CH3NH3PbI3 for Mesoscopic Perovskite Solar Cells Chem. Commun., 2014, DOI) which is subsequently converted to a Perovskite coating through reaction with suitable precursor materials. Whilst a thin layer can be formed by this method, the deposition kinetics of metal compounds on the substrate to produce a suitably thick layer is slow (typically more than 1 hour). Commercial scale-up of these types of processes is therefore limited.
It is therefore desirable to provide a new and/or improved process or method of forming a photoactive layer of an optoelectronic device.